Outlook users suddenly start sending to X.500 addresses and receive a delivery failure in response - why?

Question

Outlook

Just in case: recently there was a failure to restore the server and databases from a backup. The recovery was successful, the mail goes. But.
A small part of users have the following problem: taking the user's address from the cache and sending it to it (well, the user enters "Vasya" and outlook takes the contact "Vasya Pupkin" from the cache) they receive a delivery failure in response - the user does not exist. If you look at the get-messagetrackinglog, you can see that the sending in this case goes to the X.500 address. Moreover, adding the X.500 address (copying from the LegacyExchangeDN field) to the list of mailbox addresses does not solve the problem. The problem is solved only by updating the address book on the client (if outlook 2007) or deleting the contact from the cache by hand (if outlook 2013). The problem is that there are a lot of users and it’s hard for everyone to fix it with their hands. What caused such a bug? LegacyExchangeDN after all matches.